In the case of lip seals used to seal aqueous fluids, such as LLC (long life coolant), the low lubricity of sliding surfaces causes significant abrasion due to sliding and leads to high heat generation. LLC containing organic acid, particularly 2-ethylhexanoic acid, causes significant softening and swelling of the lip seal, thus reducing sealing properties. Moreover, LLC containing phosphate or silicate leads to accumulation of phosphoric acid compounds or silicate compounds on the shaft and formation of a gap between the shaft and the lip seal, thus reducing sealing properties.
In order to improve lubricity in such a case, Patent Document 1 discloses a water pump sealing device using a lubricating grease for a lip lubricating.
More specifically, the water pump sealing device, which is fitted between a housing and a shaft so that a sealing fluid, such as LLC, does not leak from the inside of the device to the outside, can improve the lubricity of a seal lip sliding part to prevent the seal lip from being prematurely worn by slidable movement. In one structure, the sealing device comprises a plurality of seal lips and a grease-impregnating member impregnated with a lip-lubricating grease, wherein the grease-impregnating member is arranged in a space part between a plurality of the seal lips. In another structure, the sealing device comprises a sleeve secured to a shaft, first and second seal lips in sliding contact with the sleeve, a backup ring, and a grease-impregnating member, wherein the grease-impregnating member is arranged in a space that lies between the both seal lips and that is a radical direction gap part where the sleeve and the backup ring are radially opposed to each other.
Meanwhile, Patent Document 2 proposes a hydrogenated nitrile rubber composition comprising 100 parts by weight of hydrogenated nitrile rubber, and a total amount of about 120 parts by weight or more of carbon black and other filler, such as graphite, carbon fiber, silica, talc, clay, PTFE powder, activated calcium carbonate, or calcium silicate. The hydrogenated nitrile rubber composition provides a crosslinked product having a thermal conductivity at 20° C. of 0.4 W/m·k or more and a 50% modulus of 14 MPa or more. Patent Document 2 indicates that the crosslinked product can significantly reduce the amount of heat generated during sliding, and thus can be suitably used as a sliding or high-pressure sealing material. However, when the crosslinked product is used as a lip seal to seal an aqueous-based fluid, such as LLC, the above-mentioned various properties inevitably decrease.
Moreover, Patent Document 3 proposes an NBR composition comprising 100 parts by weight of NBR, 1 to 150 parts by weight of white carbon (silica), and 0.5 to 50 parts by weight of an inorganic compound having an average particle diameter of 2 μm or less and a Mohs hardness of 6 or more, such as aluminum oxide, silicon carbide, tungsten carbide, zirconium oxide, iron oxide, titanium oxide, quartz powder, titanium nitride, titanium carbide, or zirconium carbide. Patent Document 3 indicates that a vulcanization-molded product of the NBR composition can achieve long life and energy saving when sealing materials are used in a sliding part, without impairing abrasion resistance. However, when the vulcanization-molded product is used as a lip seal to seal an aqueous-based fluid, such as LLC, the above-mentioned various properties inevitably decrease.